HERA: a super-microscope for the proton at the energy frontier

A decisive step in the understanding of the proton structure was the arrival of the unique electron-proton collider HERA (hadron electron ring accelerator) at DESY in Hamburg (Germany). The centre-of-mass energy available at HERA of up to 320 GeV results from collision of 27,5 GeV electrons of positrons with 920 GeV protons. The collision of an electron with a proton is generally mediated by a photon emitted from the incident electron. The distance probed into the proton by this photon is of 10-18 order. At such small distances, the photon can directly interact with the quarks of the proton. The reactions reveal how the quarks compose the proton, via the measurement of the fraction x of the proton's momentum carried by the struck quark. More and more quarks are found in the proton as x decreases. QCD explains this intriguing facts as quarks are produced by gluons, which are abundant in the low x regime and fluctuate to quark - anti-quark pairs. The proton structure cannot be predicted in contemporary physics. It is a consequence of a peculiar property of the strong force: the interaction between 2 quarks becomes stronger as they get separated (asymptotic freedom). Given the size of the proton the typical distance between 2 quarks is such that the coupling constant is large. Therefore the usual calculation methods based on perturbation development are not applicable. Globally HERA results confirm that known elementary particles are indeed point-like with a size in any case smaller than the experimental resolution of 10-18 m. (A.C.)